1. Field of the invention
The present invention relates to a method for driving a turn detector circuit which detects the turning of a step motor used in timepieces such as wristwatches.
2. Description of the Prior Art
FIGS. 1, 2 and 3 respectively illustrate conventional driving and detecting circuits, a step motor, and a timing chart thereof. In operation, at a timing T1, a drive pulse is applied to an excitation coil (hereinafter simply referred to as coil) 9. A p-type MOSFET 1 (hereinafter simply referred to as Tr) and an n-type MOSFET 6 (hereinafter simply referred to as Tr) only are rendered conductive, and an electric current flows from the power source through a path indicated by arrow 14. At a timing T2 for detecting the turning or angular advancement of the motor rotor, Tr 6 is conductive at all times, and Tr 2 and Tr 3 are rendered conductive and nonconductive alternately. Namely, closed inpedance loops 15 and 16 are alternately established, and a voltage VRS is induced in the coil due to the motion of a rotor 17 as represented by a voltage at point 4 in FIG. 3. The voltage VRS is usually about 2.0 volts (current flows into VDD via a parasitic diode in the IC, and is clamped to a forward voltage of VDD+ parasitic diode) when the rotor turns, with VSS being set at 0 volts, and is from 0.3 to 1.2 volts when the rotor does not turn. The difference between VRS when the rotor turns and that when the rotor does not turn is detected by comparators 10 and 11, thereby to detect the turn.
The above method for driving a turn detector circuit to detect whether a rotor turns or not is exemplified in U.S. Pat. No. 4,326,278 issued Apr. 20, 1982 to Shida et al.
The voltage VRS, however, is affected by quite many factors such as the shapes and materials of the rotor 17 and stator 18, the inductance and resistance of the coil 9, the resistances of resistors 12 and 13, the switching timing of Tr's 2 and 6, the stray capacitance at both ends 4 and 8 of the coil 9, and the switching timing T2. The voltage VRS varies due to these factors, and the rotor 17 is often detected to be not turning when it actually is turning, or is detected, to be turning when it is not turning. This is a serious problem causing the consumption of electric power to increase or causing the watch to run slow.
The rotors, stators and coils are typically formed in small sizes in accordance with the recent trend toward constructing the timepieces in compact sizes thereby resulting, however, in making the voltage VRS higher. Namely, though the rotor is not really turning, it is sometimes detected that the rotor is turning, and consequently the displayed time lags behind the real or actual time. Among the many factors that affect the voltages VRS, the switching timings of Tr's 2, 3 and Tr's 6, 7 during the timing T2 have heretofore been so selected that Tr 3 or 7 is rendered conductive just when Tr 2 or 6 is rendered non-conductive as shown in FIG. 4A. However, though the switching timing can be selected just as mentioned above, the time is delayed by small amounts due to wiring capacity or capacitance and gate capacity or capacitance in the IC. There will be no problems when the signals applied to the gates of Tr's 2 and 3 are delayed by an equal amount, or when the signal applied to Tr 2 is delayed more than the signal applied to Tr 3 as shown in FIG. 4C. However, a large voltage VRS is induced when the signal applied to Tr 3 is more delayed than the signal applied to Tr 2 as shown in FIG. 4B. Namely, at the time T3 of FIG. 4B where both the Tr 2 and Tr 3 are rendered non-conductive, there is formed no closed loop, and a high voltage VRS is induced.